The designers of all forms and classes of computer systems have commonly exhibited the desire to implement higher speed systems having ever increasing amounts of immediately available random access memory. The desire for increased speed is a reflection of a number of interrelated factors, including increased computer instruction complexity, the length and complexity of application programs, and the shear quantity of data to be managed and transformed in the execution of application programs. Naturally, the immediately accessible random access memory of such systems must also have high access speed as well as the capability of storing a high capacity or density of data. This is required to ensure that data access is not the limiting factor in the performance of the overall computer system.
Certain applications, such as high resolution digital video graphics, place heightened requirements on the level of data densities and access speeds necessary to adequately accomplish their goal or function. In digital video graphics, the high densities of data are required to accurately represent the digital graphic video images. However, as the level of data utilized increases, the access speed to any particular datum must correspondingly increase to permit adequate manipulation of the image data. Further, the time period allowed for accessing and manipulating image data is severly constrained to non-screen refresh periods. Typically, such non-screen refresh periods amount to only 13% of any image access period.
In most instances where high capacity semiconductor memories are required, the use of dynamic RAM is prevalent due to its cost effectiveness. As with most digital semiconductor technologies, there have and continue to be improvements, including increasing the data density and access speed of dynamic RAM memories. Unfortunately, these continuing improvements occur only in small increments. Naturally, a substantial increase in either the data density storage capability or access speed, or preferably both, is highly desirable.
One reason for achieving only incremental increases in the data density and access speed of dynamic RAM memories is that any modification must be considered in light of all of the constraints on a dynamic memory system. In particular, such modifications must be considered in terms of their impact on, for example, the sensitive dynamic RAM sensing scheme utilized. Further, as the data density increases the need for provide for memory cell redundancy similarly increases. Any modification to enhance access speed must take into account not only the povision of redundant memory cells, but the additional routing circuitry necessary to effect the replacement of defective memory cells with their redundant counterparts. Additionally, all such modifications, as a practical matter, need be implemented so as to provide the least burden upon a designer of memory systems to obtain full benefit of the memory speed and data density enhancements in the completed system.